Thermal behaviour of pure and binary Fe(NO 3) 3·9H 2O and (NH 4) 6Mo 7O 24·4H 2O systems

Abstract

Thermal behaviour of pure ferric nitrate, ammonium molybdate and their mixtures in different ratios were investigated by means of thermal analysis (TG, DTG and DTA) techniques. Relative thermal analysis (RTA) graphical treatment of derivatographic curves of the components in the pure and binary system has been carried out as well. A series of Fe 2O 3–MoO 3 systems were prepared from pure and binary salts by heating at 350, 550, 750 and 1000 °C. The X-ray diffraction (XRD) analysis was used to characterize the phases produced from thermal treatment of investigated solids. The results revealed that pure ferric nitrate decomposed to Fe 2O 3 at 250, while pure ammonium molybdate decomposed into MoO 3 and 340 °C and then melted at 790 °C. For the binary components, crystalline ferric or molybdenum oxides were detected beside ferric molybdate Fe 2(MoO 4) 3 phase starting from 350 °C. Fe 2(MoO 4) 3 phase was formed as a result of solid-solid interactions between the produced oxides. The thermal stability of the formed compound was significantly affected by the composition of the mixture and treatment temperature. The presence of two-component solids in the binary systems affected the thermal decomposition of their individual salt and affected their physical and chemical behaviour. The catalytic activity of the obtained pure and mixed oxides was measured using the decomposition of hydrogen peroxide reaction as a model reaction at 20–50 °C. It was found that the mixed oxide solids had catalytic activity higher than single oxides thermally treated at 350 and 550 °C. This is attributed to the increase in the concentration of active sites via creation of new ion pairs in case of binary systems. The rise in calcination temperature up to 750 and 1000 °C brought about drastic decrease in the activity of all solids because of changing catalyst composition and/or sintering process. The activation energies of H 2O 2 decomposition were determined for pure and mixed solids. The results obtained were discussed in light of induced changes in chemical composition and treatment temperature.